Remember the
magnitude of the force felt by a charge q
moving with velocity v through
a magnetic field B is:

As stated
previously, the direction of the force is perpendicular
to both v and B.
Of course, if both v and B
are pointed in the plane of the screen, F
could be either into or out of the screen to be
perpendicular to both vectors. One uses the right-hand
rule to determine the direction of F.

To find the
direction of F use your right
hand and:

Point
your thumb into the direction of v.

Point
your fingers into the direction of B.

The
direction of the force will be out of your palm.
Your fingers will curl into the direction of the
force.

If the charge is negative, one must
remember the direction of the force will be opposite. To
calculate the force on a segment of wire, use the
direction of the current I instead of v.

If you had used your left hand the
force would have been directed oppositely. It seems as if
this violates fundamental American principles of
equality. However, we will learn that the direction of B
is also determined through a right-hand rule, and the
application of two right-hand rules to get to something
meaningful (the force) means that two left-hand rules
would have given the same result. Thus the laws of
electromagnetism do not favor right-handed vs.
left-handed people, they only favor consistency. If a
physical result depended on the right-handed rule that
would constitute violation of parity. This
indeed occurs in weak decays which will be studied at the
end of this course. In weak decays, if one considers a
nucleus where the charge rotates as your fingers wrap
around your right hand, the emitted electrons all go
along the direction of your thumb. Curiously enough, if
one did the same experiment with antimatter, one would
have to use one's left hand to find the direction of the
emitted antielectrons.